Rare earth permanent magnets and their preparation

ABSTRACT

A sintered magnet body (R a T 1   b M c B d ) coated with a powder mixture of an intermetallic compound (R 1   i M 1   j , R 1   x T 2   y M 1   z , R 1   i M 1   j H k ), alloy (M 1   d M 2   e ) or metal (M 1 ) powder and a rare earth (R 2 ) oxide is diffusion treated. The R 2  oxide is partially reduced during the diffusion treatment, so a significant amount of R 2  can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries. The coercive force is increased while minimizing a decline of remanence.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.15/454,433, filed on Mar. 9, 2017, and wherein U.S. application Ser. No.15/454,433 is a Divisional of U.S. patent application Ser. No.13/461,043 filed on May 1, 2012, which is a non-provisional applicationwhich claims priority under 35 U.S.C. § 119(a) on Japanese PatentApplication Nos. 2011-102787 and 2011-102789 filed in Japan on May 2,2011 and May 2, 2011, respectively, the entire contents of which arehereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an R—Fe—B permanent magnet having an enhancedcoercive force with a minimal decline of remanence, and a method forpreparing the same by coating a sintered magnet body with a mixture ofan intermetallic compound, alloy or metal powder and a rare earth oxideand heat treating the coated body for diffusion.

BACKGROUND ART

By virtue of excellent magnetic properties, Nd—Fe—B permanent magnetsfind an ever increasing range of application. The recent challenge tothe environmental problem has expanded the application range of thesemagnets from household electric appliances to industrial equipment,electric automobiles and wind power generators. It is required tofurther improve the performance of Nd—Fe—B magnets.

Indexes for the performance of magnets include remanence (or residualmagnetic flux density) and coercive force. An increase in the remanenceof Nd—Fe—B sintered magnets can be achieved by increasing the volumefactor of Nd₂Fe₁₄B compound and improving the crystal orientation. Tothis end, a number of modifications have been made. For increasingcoercive force, there are known different approaches including grainrefinement, the use of alloy compositions with greater Nd contents, andthe addition of coercivity enhancing elements such as Al and Ga. Thecurrently most common approach is to use alloy compositions having Dy orTb substituted for part of Nd.

It is believed that the coercivity creating mechanism of Nd—Fe—B magnetsis the nucleation type wherein nucleation of reverse magnetic domains atgrain boundaries governs a coercive force. In general, a disorder ofcrystalline structure occurs at the grain boundary or interface. If adisorder of crystalline structure extends several nanometers in a depthdirection near the interface of grains of Nd₂Fe₁₄B compound which is theprimary phase of the magnet, then it incurs a lowering ofmagnetocrystalline anisotropy and facilitates formation of reversemagnetic domains, reducing a coercive force (see Non-Patent Document 1).Substituting Dy or Tb for some Nd in the Nd₂Fe₁₄B compound increases theanisotropic magnetic field of the compound phase so that the coerciveforce is increased. When Dy or Tb is added in an ordinary way, however,a loss of remanence is unavoidable because Dy or Tb substitution occursnot only near the interface of the primary phase, but even in theinterior of the primary phase. Another problem arises in that amounts ofexpensive Tb and Dy must be used.

Besides, a number of attempts have been made for increasing the coerciveforce of Nd—Fe—B magnets. One exemplary attempt is a two-alloy method ofpreparing an Nd—Fe—B magnet by mixing two powdered alloys of differentcomposition and sintering the mixture. Specifically, a powder of alloy Aconsisting of R₂Fe₁₄B primary phase wherein R is mainly Nd and Pr, and apowder of alloy B containing various additive elements including Dy, Tb,Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb are mixed together. Thisis followed by fine pulverization, molding in a magnetic field,sintering, and aging treatment whereby the Nd—Fe—B magnet is prepared.The sintered magnet thus obtained produces a high coercive force whileminimizing a decline of remanence because Dy and Tb are absent at thecenter of R₂Fe₁₄B compound primary phase grains and instead, theadditive elements like Dy and Tb are localized near grain boundaries(see Patent Documents 1 and 2). In this method, however, Dy and Tbdiffuse into the interior of primary phase grains during the sinteringso that the layer where Dy and Tb are localized near grain boundarieshas a thickness equal to or more than about 1 micrometer, which issubstantially greater than the depth where nucleation of reversemagnetic domains occurs. The results are still not fully satisfactory.

Recently, there have been developed several processes of diffusingcertain elements from the surface to the interior of a R—Fe—B sinteredbody for improving magnet properties. In one exemplary process, a rareearth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on thesurface of Nd—Fe—B magnet using an evaporation or sputtering technique,followed by heat treatment, as described in Patent Documents 3 to 5 andNon-Patent Documents 2 and 3. Another exemplary process involvesapplying a powder of rare earth inorganic compound such as fluoride oroxide onto the surface of a sintered body and heat treatment asdescribed in Patent Document 6. With these processes, the elements(e.g., Dy and Tb) disposed on the sintered body surface pass throughgrain boundaries in the sintered body structure and diffuse into theinterior of the sintered body during the heat treatment. As aconsequence, Dy and Tb can be enriched in a very high concentration atgrain boundaries or near grain boundaries within sintered body primaryphase grains. As compared with the two-alloy method describedpreviously, these processes produce an ideal morphology. Since themagnet properties reflect the morphology, a minimized decline ofremanence and an increase of coercive force are accomplished. However,the processes utilizing evaporation or sputtering have many problemsassociated with units and steps when practiced on a mass scale andsuffer from poor productivity.

Besides the foregoing methods, Patent Document 6 discloses a methodcomprising coating a surface of a sintered body with a powdered rareearth inorganic compound such as fluoride or oxide and heat treatment,and Patent Document 8 discloses a method comprising mixing an Al, Cu orZn powder with a fluoride, coating a magnet with the mixture, and heattreatment. These methods are characterized by a very simple coating stepand a high productivity. Specifically, since the coating step is carriedout by dispersing a non-metallic inorganic compound powder in water,immersing a magnet in the dispersion and drying, the step is simple ascompared with sputtering and evaporation. Even when a heat treatmentfurnace is packed with a large number of magnet pieces, the magnetpieces are not fused together during heat treatment. This leads to ahigh productivity. However, since Dy or Tb diffuses through substitutionreaction between the powder and the magnet component, it is difficult tointroduce a substantial amount of Dy or Tb into the magnet.

Further Patent Document 7 discloses coating of a magnet body with amixture of an oxide or fluoride of Dy or Tb and calcium or calciumhydride powder, followed by heat treatment. During the heat treatment,once Dy or Tb is reduced utilizing calcium reducing reaction, Dy or Tbis diffused. The method is advantageous for introducing a substantialamount of Dy or Tb into the magnet, but less productive because thecalcium or calcium hydride powder needs careful handling.

Patent Documents 9 to 13 disclose coating of the sintered body surfacewith a metal alloy instead of a rare earth inorganic compound powdersuch as fluoride or oxide, followed by heat treatment. The method ofcoating with only metal alloy has the drawback that it is difficult tocoat the metal alloy onto the magnet surface in a large and uniformcoating weight. In Patent Documents 14 and 15, a metal powder containingDy and/or Tb is diffused into the mother alloy. The oxygen concentrationof the mother alloy is restricted below 0.5% by weight, and the rareearth-containing metal powder is closely contacted with the mother alloyby a barrel painting technique of oscillating impact media within abarrel for agitation. Diffusion takes place under these conditions.However, this method requires many steps as compared with the method ofcoating a mother alloy magnet with a dispersion of a powder mixture ofan intermetallic compound and a rare earth oxide in a solvent. Themethod is time consuming and is not industrially useful.

CITATION LIST

Patent Document 1: JP 1820677

Patent Document 2: JP 3143156

Patent Document 3: JP-A 2004-296973

Patent Document 4: JP 3897724

Patent Document 5: JP-A 2005-11973

Patent Document 6: JP 4450239

Patent Document 7: JP 4548673

Patent Document 8: JP-A 2007-287874

Patent Document 9: JP 4656323

Patent Document 10: JP 4482769

Patent Document 11: JP-A 2008-263179

Patent Document 12: JP-A 2009-289994

Patent Document 13: JP-A 2010-238712

Patent Document 14: WO 2008/032426

Patent Document 15: WO 2008/139690

Non-Patent Document 1: K. D. Durst and H. Kronmuller, “THE COERCIVEFIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism andMagnetic Materials, 68 (1987), 63-75

Non-Patent Document 2: K.T. Park, K. Hiraga and M. Sagawa, “Effect ofMetal-Coating and Consecutive Heat Treatment on Coercivity of ThinNd—Fe—B Sintered Magnets,” Proceedings of the Sixteen InternationalWorkshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257(2000)

Non-Patent Document 3: K. Machida, et al., “Grain Boundary Modificationof Nd—Fe—B Sintered Magnet and Magnetic Properties,” Proceedings of 2004Spring Meeting of the Powder & Powder Metallurgy Society, p. 202

SUMMARY OF INVENTION

An object of the invention is to provide an R—Fe—B sintered magnet whichis prepared by coating a sintered magnet body with a powder mixture ofan intermetallic compound, alloy or metal powder and a rare earth oxideand effecting diffusion treatment and which magnet features efficientproductivity, excellent magnetic performance, a minimal amount of Tb orDy used, an increased coercive force, and a minimized decline ofremanence. Another object is to provide a method for preparing the same.

Regarding the surface coating of an R—Fe—B sintered body with a rareearth oxide which is the best from the aspect of productivity, theinventors attempted to increase the diffusion amount. The inventors havediscovered that when a mixture of an oxide containing a rare earthelement such as Dy or Tb and an intermetallic compound or metal powderis used for coating, a significant amount of Dy or Tb can be introducednear interfaces of primary phase grains within the magnet through thepassages in the form of grain boundaries, as compared with the method ofeffecting heat treatment after coating with a rare earth inorganiccompound powder such as fluoride or oxide, because the oxide ispartially reduced during heat treatment. As a consequence, the coerciveforce of the magnet is increased while minimizing a decline ofremanence. Additionally, the process is improved in productivity overthe prior art processes. The invention is predicated on this discovery.

The invention provides rare earth permanent magnets and methods forpreparing the same, as defined below.

[1] A method for preparing a rare earth permanent magnet, comprising thesteps of:

disposing a powder mixture on a surface of a sintered magnet body havingthe composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(i)M¹ _(j) whereinR¹ is at least one element selected from rare earth elements inclusiveof Y and Sc, M¹ is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge,Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, “i” and “j”indicative of atomic percent are in the range: 15<j≤99, the balance ofi, and i+j=100, containing at least 70% by volume of an intermetalliccompound phase, and having an average particle size of up to 500 μm, andat least 10% by weight of an R² oxide wherein R² is at least one elementselected from rare earth elements inclusive of Y and Sc, having anaverage particle size of up to 100 μm, and

heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, for causing the elements R¹, R² and M¹ in the powder mixtureto diffuse to grain boundaries in the interior of the sintered magnetbody and/or near grain boundaries within the sintered magnet bodyprimary phase grains.

[2] A method for preparing a rare earth permanent magnet, comprising thesteps of:

disposing a powder mixture on a surface of a sintered magnet body havingthe composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(i)M¹ _(j)H_(k)wherein R¹ is at least one element selected from rare earth elementsinclusive of Y and Sc, M¹ is at least one element selected from thegroup consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga,Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen,“i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99,0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% byvolume of an intermetallic compound phase, and having an averageparticle size of up to 500 μm, and at least 10% by weight of an R² oxidewherein R² is at least one element selected from rare earth elementsinclusive of Y and Sc, having an average particle size of up to 100 μm,and

heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, for causing the elements R¹, R², and M¹ in the powder mixtureto diffuse to grain boundaries in the interior of the sintered magnetbody and/or near grain boundaries within the sintered magnet bodyprimary phase grains.

[3] The method of [1] or [2] wherein the heat treating step includesheat treatment at a temperature from 200° C. to (Ts−10)° C. for 1 minuteto 30 hours wherein Ts represents the sintering temperature of thesintered magnet body.[4] The method of any one of [1] to [3] wherein the disposing stepincludes dispersing the powder mixture in an organic solvent or water,immersing the sintered magnet body in the resulting slurry, taking upthe sintered magnet body, and drying for thereby covering the surface ofthe sintered magnet body with the powder mixture.[5] A method for preparing a rare earth permanent magnet, comprising thesteps of:

disposing a powder mixture on a surface of a sintered magnet body havingthe composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(x)T² _(y)M¹ _(z)wherein R¹ is at least one element selected from rare earth elementsinclusive of Y and Sc, T² is one or both of Fe and Co, M¹ is at leastone element selected from the group consisting of Al, Si, C, P, Ti, V,Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb,and Bi, x, y and z indicative of atomic percent are in the range:5≤x≤85, 15<z≤95, x+z<100, the balance of y, y>0, and x+y+z=100,containing at least 70% by volume of an intermetallic compound phase,and having an average particle size of up to 500 μm, and at least 10% byweight of an R² oxide wherein R² is at least one element selected fromrare earth elements inclusive of Y and Sc, having an average particlesize of up to 100 μm, and

heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, for causing the elements R¹, R², M¹ and T² in the powdermixture to diffuse to grain boundaries in the interior of the sinteredmagnet body and/or near grain boundaries within the sintered magnet bodyprimary phase grains.

[6] The method of [5] wherein the heat treating step includes heattreatment at a temperature from 200° C. to (Ts−10)° C. for 1 minute to30 hours wherein Ts represents the sintering temperature of the sinteredmagnet body.[7] The method of [5] or [6] wherein the disposing step includesdispersing the powder mixture in an organic solvent or water, immersingthe sintered magnet body in the resulting slurry, taking up the sinteredmagnet body, and drying for thereby covering the surface of the sinteredmagnet body with the powder mixture.[8] The method of any one of [1] to [7] wherein the sintered magnet bodyhas a shape including a minimum portion with a dimension equal to orless than 20 mm.[9] A rare earth permanent magnet, which is prepared by disposing apowder mixture on a surface of a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(i)M¹ _(i) whereinR¹ is at least one element selected from rare earth elements inclusiveof Y and Sc, M¹ is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge,Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, “i” and “j”indicative of atomic percent are in the range: 15<j≤99, the balance ofi, and i+j=100, containing at least 70% by volume of an intermetalliccompound phase, and having an average particle size of up to 500 μm, andat least 10% by weight of an R² oxide wherein R² is at least one elementselected from rare earth elements inclusive of Y and Sc, having anaverage particle size of up to 100 μm, and heat treating the sinteredmagnet body having the powder mixture disposed on its surface at atemperature lower than or equal to the sintering temperature of thesintered magnet body in vacuum or in an inert gas, wherein

the elements R¹, R² and M¹ in the powder mixture are diffused to grainboundaries in the interior of the sintered magnet body and/or near grainboundaries within the sintered magnet body primary phase grains so thatthe coercive force of the rare earth permanent magnet is increased overthe original sintered magnet body.

[10] A rare earth permanent magnet, which is prepared by disposing apowder mixture on a surface of a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(i)M¹ _(j)H_(k)wherein R¹ is at least one element selected from rare earth elementsinclusive of Y and Sc, M¹ is at least one element selected from thegroup consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga,Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, H is hydrogen,“i,” “j” and “k” indicative of atomic percent are in the range: 15<j≤99,0<k≤(i×2.5), the balance of i, and i+j+k=100, containing at least 70% byvolume of an intermetallic compound phase, and having an averageparticle size of up to 500 μm, and at least 10% by weight of an R² oxidewherein R² is at least one element selected from rare earth elementsinclusive of Y and Sc, having an average particle size of up to 100 μm,and heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, wherein

the elements R¹, R² and M¹ in the powder mixture are diffused to grainboundaries in the interior of the sintered magnet body and/or near grainboundaries within the sintered magnet body primary phase grains so thatthe coercive force of the rare earth permanent magnet is increased overthe original sintered magnet body.

[11] A rare earth permanent magnet, which is prepared by disposing apowder mixture on a surface of a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition R¹ _(x)T² _(y)M¹ _(z)wherein R¹ is at least one element selected from rare earth elementsinclusive of Y and Sc, T² is one or both of Fe and Co, M¹ is at leastone element selected from the group consisting of Al, Si, C, P, Ti, V,Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb,and Bi, x, y and z indicative of atomic percent are in the range:5≤x≤85, 15<z≤95, x+z<100, the balance of y, y>0, and x+y+z=100,containing at least 70% by volume of an intermetallic compound phase,and having an average particle size of up to 500 μm, and at least 10% byweight of an R² oxide wherein R² is at least one element selected fromrare earth elements inclusive of Y and Sc, having an average particlesize of up to 100 μm, and heat treating the sintered magnet body havingthe powder mixture disposed on its surface at a temperature lower thanor equal to the sintering temperature of the sintered magnet body invacuum or in an inert gas, wherein

the elements R¹, R², M¹ and T² in the powder mixture are diffused tograin boundaries in the interior of the sintered magnet body and/or neargrain boundaries within the sintered magnet body primary phase grains sothat the coercive force of the rare earth permanent magnet is increasedover the original sintered magnet body.

[12] A method for preparing a rare earth permanent magnet, comprisingthe steps of:

disposing a powder mixture on a surface of a sintered magnet body havingthe composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition M¹ _(d)M² _(e) whereinM¹ and M² each are at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge,Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M¹ and M² aredifferent, “d” and “e” indicative of atomic percent are in the range:0.1≤e≤99.9, the balance of d, and d+e=100, containing at least 70% byvolume of an intermetallic compound phase, and having an averageparticle size of up to 500 μm, and at least 10% by weight of an R² oxidewherein R² is at least one element selected from rare earth elementsinclusive of Y and Sc, having an average particle size of up to 100 μm,and

heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, for causing the elements R², M¹ and M² in the powder mixtureto diffuse to grain boundaries in the interior of the sintered magnetbody and/or near grain boundaries within the sintered magnet bodyprimary phase grains.

[13] A method for preparing a rare earth permanent magnet, comprisingthe steps of:

disposing a powder mixture on a surface of a sintered magnet body havingthe composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an M¹ powder wherein M¹ is at least one element selected fromthe group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn,Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, having anaverage particle size of up to 500 μm, and at least 10% by weight of anR² oxide wherein R² is at least one element selected from rare earthelements inclusive of Y and Sc, having an average particle size of up to100 μm, and

heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, for causing the elements R² and M¹ in the powder mixture todiffuse to grain boundaries in the interior of the sintered magnet bodyand/or near grain boundaries within the sintered magnet body primaryphase grains.

[14] The method of [12] or [13] wherein the heat treating step includesheat treatment at a temperature from 200° C. to (Ts−10)° C. for 1 minuteto 30 hours wherein Ts represents the sintering temperature of thesintered magnet body.[15] The method of any one of [12] to [14] wherein the disposing stepincludes dispersing the powder mixture in an organic solvent or water,immersing the sintered magnet body in the resulting slurry, taking upthe sintered magnet body, and drying for thereby covering the surface ofthe sintered magnet body with the powder mixture.[16] The method of any one of [12] to [15] wherein the sintered magnetbody has a shape including a minimum portion with a dimension equal toor less than 20 mm.[17] A rare earth permanent magnet, which is prepared by disposing apowder mixture on a surface of a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an alloy powder having the composition M¹ _(d)M² _(e) whereinM¹ and M² each are at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge,Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, M¹ and M² aredifferent, “d” and “e” indicative of atomic percent are in the range:0.1≤e≤99.9, the balance of d, and d+e=100, containing at least 70% byvolume of an intermetallic compound phase, and having an averageparticle size of up to 500 μm, and at least 10% by weight of an R² oxidewherein R² is at least one element selected from rare earth elementsinclusive of Y and Sc, having an average particle size of up to 100 μm,and heat treating the sintered magnet body having the powder mixturedisposed on its surface at a temperature lower than or equal to thesintering temperature of the sintered magnet body in vacuum or in aninert gas, wherein

the elements R², M¹ and M² in the powder mixture are diffused to grainboundaries in the interior of the sintered magnet body and/or near grainboundaries within the sintered magnet body primary phase grains so thatthe coercive force of the rare earth permanent magnet is increased overthe original sintered magnet body.

[18] A rare earth permanent magnet, which is prepared by disposing apowder mixture on a surface of a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at least one elementselected from rare earth elements inclusive of Y and Sc, T¹ is one orboth of Fe and Co, M is at least one element selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c” and“d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an M¹ powder wherein M¹ is at least one element selected fromthe group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn,Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, having anaverage particle size of up to 500 μm, and at least 10% by weight of anR² oxide wherein R² is at least one element selected from rare earthelements inclusive of Y and Sc, having an average particle size of up to100 μm, and heat treating the sintered magnet body having the powdermixture disposed on its surface at a temperature lower than or equal tothe sintering temperature of the sintered magnet body in vacuum or in aninert gas, wherein

the elements R² and M¹ in the powder mixture are diffused to grainboundaries in the interior of the sintered magnet body and/or near grainboundaries within the sintered magnet body primary phase grains so thatthe coercive force of the rare earth permanent magnet is increased overthe original sintered magnet body.

ADVANTAGEOUS EFFECTS OF INVENTION

When a mixture of an oxide containing a rare earth element such as Dy orTb and an intermetallic compound or metal powder is used for coating,the oxide is partially reduced during subsequent heat treatment. Thus asignificant amount of the rare earth element such as Dy or Tb can beintroduced near interfaces of primary phase grains within the magnetthrough the passages in the form of grain boundaries, as compared withthe method of effecting heat treatment after coating with a rare earthinorganic compound powder such as fluoride or oxide. As a consequence,the coercive force of the magnet is increased while minimizing a declineof remanence. Additionally, the process is improved in productivity overthe prior art processes. The R—Fe—B sintered magnet exhibits excellentmagnetic performance, an increased coercive force, and a minimal declineof remanence, despite a minimal amount of Tb or Dy used.

DESCRIPTION OF EMBODIMENTS

Briefly stated, an R—Fe—B sintered magnet is prepared according to theinvention by applying a powder mixture of an intermetalliccompound-based alloy powder and a rare earth oxide or metal powder ontoa sintered magnet body and effecting diffusion treatment. The resultantmagnet has advantages including excellent magnetic performance and aminimal amount of Tb or Dy used.

The mother material used herein is a sintered magnet body having thecomposition R_(a)T¹ _(b)M_(c)B_(d), which is sometimes referred to as“mother sintered body.” Herein R is one or more elements selected fromrare earth elements inclusive of yttrium (Y) and scandium (Sc),specifically from among Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Yb, and Lu. The rare earth elements inclusive of Sc and Y accountfor 12 to 20 atomic percent (at %), and preferably 13 to 18 at % of thesintered magnet body, differently stated, 12 a 20, preferably 13≤a≤18.Preferably the majority of R is Nd and/or Pr. Specifically Nd and/or Praccounts for 50 to 100 at %, more preferably 70 to 100 at % of the rareearth elements. T¹ is one or both of iron (Fe) and cobalt (Co). M is oneor more elements selected from the group consisting of Al, Si, C, P, Ti,V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W,Pb, and Bi and accounts for 0 to 10 at %, and preferably 0 to 5 at % ofthe sintered magnet body, differently stated, 0≤c≤10, preferably 0≤c≤5.B is boron and accounts for 4 to 7 at % of the sintered magnet body(4≤d≤7). Particularly when B is 5 to 6 at % (5≤d≤6), a significantimprovement in coercive force is achieved by diffusion treatment. Thebalance consists of T¹. Preferably T¹ accounts for 60 to 84 at %, morepreferably 70 to 82 at % of the sintered magnet body, differentlystated, 60≤b≤84, preferably 70≤b≤82. The subscripts “a,” “b,” “c” and“d” indicative of atomic percent meet a+b+c+d=100.

The alloy for the mother sintered magnet body is prepared by meltingmetal or alloy feeds in vacuum or an inert gas atmosphere, preferablyargon atmosphere, and casting the melt into a flat mold or book mold orstrip casting. A possible alternative is a so-called two-alloy processinvolving separately preparing an alloy approximate to the R₂Fe₁₄Bcompound composition constituting the primary phase of the relevantalloy and a rare earth-rich alloy serving as a liquid phase aid at thesintering temperature, crushing, then weighing and mixing them. Notably,the alloy approximate to the primary phase composition is subjected tohomogenizing treatment, if necessary, for the purpose of increasing theamount of the R₂Fe₁₄B compound phase, since primary crystal α-Fe islikely to be left depending on the cooling rate during casting and thealloy composition. The homogenizing treatment is a heat treatment at 700to 1,200° C. for at least one hour in vacuum or in an Ar atmosphere.Alternatively, the alloy approximate to the primary phase compositionmay be prepared by the strip casting technique. To the rare earth-richalloy serving as a liquid phase aid, the melt quenching and stripcasting techniques are applicable as well as the above-described castingtechnique.

The alloy is generally crushed or coarsely ground to a size of 0.05 to 3mm, especially 0.05 to 1.5 mm. The crushing step uses a Brown mill orhydrogen decrepitation, with the hydrogen decrepitation being preferredfor those alloys as strip cast. The coarse powder is then finely dividedto an average particle size of 0.2 to 30 μm, especially 0.5 to 20 μm,for example, on a jet mill using high-pressure nitrogen.

The fine powder is compacted on a compression molding machine under amagnetic field. The green compact is then placed in a sintering furnacewhere it is sintered in vacuum or in an inert gas atmosphere usually ata temperature of 900 to 1,250° C., preferably 1,000 to 1,100° C. Thesintered block thus obtained contains 60 to 99% by volume, preferably 80to 98% by volume of the tetragonal R₂Fe₁₄B compound as the primaryphase, with the balance being 0.5 to 20% by volume of a rare earth-richphase and 0.1 to 10% by volume of at least one compound selected fromamong rare earth oxides, and carbides, nitrides and hydroxides ofincidental impurities, and mixtures or composites thereof.

The resulting sintered magnet block may be machined or worked into apredetermined shape. In the invention, the elements (including R¹, R²,M¹, M² and T²) which are to be diffused into the sintered magnet bodyinterior are supplied from the sintered magnet body surface. Thus, if aminimum portion of the sintered magnet body has too large a dimension,the objects of the invention are not achievable. For this reason, theshape includes a minimum portion having a dimension equal to or lessthan 20 mm, and preferably equal to or less than 10 mm, with the lowerlimit being equal to or more than 0.1 mm. The sintered body includes amaximum portion whose dimension is not particularly limited, with themaximum portion dimension being desirably equal to or less than 200 mm.

According to the invention, a diffusion powder selected from thefollowing powder mixtures (i) to (iv) is disposed on the sintered magnetbody before diffusion treatment is carried out.

-   (i) a powder mixture of an alloy of the composition R¹ _(i)M¹ _(j)    containing at least 70% by volume of a rare earth intermetallic    compound phase and an R² oxide-   (ii) a powder mixture of an alloy of the composition R¹ _(i)M¹    _(j)H_(k) containing at least 70% by volume of a rare earth    intermetallic compound phase and an R² oxide-   (iii) a powder mixture of an alloy of the composition R¹ _(x)T²    _(y)M¹ _(z) containing at least 70% by volume of a rare earth    intermetallic compound phase and an R² oxide-   (iv) a powder mixture of an alloy of the composition M¹ _(d)M² _(e)    containing at least 70% by volume of an intermetallic compound phase    and an R² oxide-   (v) a powder mixture of a metal M¹ and an R² oxide

The alloy which is often referred to as “diffusion alloy” is in powderform having an average particle size of less than or equal to 500 μm.The R² oxide wherein R² is one or more elements selected from rare earthelements inclusive of Y and Sc is in powder form having an averageparticle size of less than or equal to 100 μm. The powder mixtureconsists of the diffusion alloy and at least 10% by weight of the R²oxide. The powder mixture is disposed on the surface of the sinteredmagnet body. The sintered magnet body having the powder mixture disposedon its surface is heat treated at a temperature lower than or equal tothe sintering temperature of the sintered magnet body in vacuum or in aninert gas, whereby the oxide in admixture with the (rare earth)intermetallic compound is partially reduced. During the heat treatment,the elements R¹, R², M¹, M² and T² in the powder mixture (selecteddepending on a particular diffusion powder used) can be diffused tograin boundaries in the interior of the sintered magnet body and/or neargrain boundaries within the sintered magnet body primary phase grains,in a more amount than achievable by the prior art methods.

Herein R¹ is one or more elements selected from rare earth elementsinclusive of Y and Sc. Preferably the majority of R¹ is Nd and/or Pr.Specifically Nd and/or Pr accounts for 1 to 100 at %, more preferably 20to 100 at % of R¹. M¹ is one or more elements selected from the groupconsisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb,Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi. T² is Fe and/or Co.

In the alloy R¹ _(i)M¹ _(j), M¹ accounts for 15 to 99 at %, preferably20 to 90 at %, differently stated, j=15 to 99, preferably j=20 to 90,with the balance of R¹ (meaning i+j=100).

In the alloy R¹ _(i)M¹ _(j)H_(k), M¹ accounts for 15 to 99 at %,preferably 20 to 90 at %, differently stated, j=15 to 99, preferablyj=20 to 90. Hydrogen (H) is present in an amount of 0<k≤(i×2.5) at %,preferably at least 0.1 at % (k≥0.1). The balance consists of R¹(meaning i+j+k=100), and R¹ is preferably present in an amount of 20 to90 at %, namely i=20 to 90.

In the alloy R¹ _(x)T² _(y)M¹ _(z), M¹accounts for 15 to 95 at %,preferably 20 to 90 at %, differently stated, z=15 to 90, preferablyz=20 to 90. R¹ accounts for 5 to 85 at %, preferably 10 to 80 at %,differently stated, x=5 to 85, preferably x=10 to 80. The sum of M¹ andR¹ is less than 100 at % (x+z<100), preferably 25 to 99.5 at % (x+y=25to 99.5). The balance consists of T² which is Fe and/or Co (meaningx+y+z=100), and y>0. Typically T² accounts for 0.5 to 75 at %,preferably 1 to 60 at %, differently stated, y=0.5 to 75, preferably y=1to 60.

In the alloy M¹ _(d)M² _(e), M¹ and M² are different from each other andeach is one or more elements selected from the group consisting of Al,Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In,Sn, Sb, Hf, Ta, W, Pb, and Bi. The subscripts d and e indicative ofatomic percent are in the range: 0.1≤e≤99.9, preferably 10≤e≤90, andmore preferably 20≤e≤80, with the balance of d.

In the M¹ metal powder, M¹ is one or more elements selected from thegroup consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga,Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi.

The diffusion alloy may contain incidental impurities such as nitrogen(N) and oxygen (O), with an acceptable total amount of such impuritiesbeing equal to or less than 4 at %, preferably equal to or less than 2at %, and more preferably equal to or less than 1 at %.

The diffusion alloy containing at least 70% by volume of theintermetallic compound phase may be prepared, like the alloy for themother sintered magnet body, by melting metal or alloy feeds in vacuumor an inert gas atmosphere, preferably argon atmosphere, and casting themelt into a flat mold or book mold. A high-frequency melting method anda strip casting method may also be employed. The alloy is then crushedor coarsely ground to a size of about 0.05 to 3 mm, especially about0.05 to 1.5 mm by means of a Brown mill or hydrogen decrepitation. Thecoarse powder is then finely divided, for example, by a ball mill,vibration mill or jet mill using high-pressure nitrogen. The smaller thepowder particle size, the higher becomes the diffusion efficiency. Thediffusion alloy containing the intermetallic compound phase, whenpowdered, preferably has an average particle size equal to or less than500 μm, more preferably equal to or less than 300 μm, and even morepreferably equal to or less than 100 μm. However, if the particle sizeis too small, then the influence of surface oxidation becomesnoticeable, and handling is dangerous. Thus the lower limit of averageparticle size is preferably equal to or more than 1 μm. As used herein,the “average particle size” may be determined as a weight averagediameter D₅₀ (particle diameter at 50% by weight cumulative, or mediandiameter) using, for example, a particle size distribution measuringinstrument relying on laser diffractometry or the like.

The M¹ metal powder may be prepared by crushing or coarsely grinding ametal mass to a size of 0.05 to 3 mm, especially 0.05 to 1.5 mm on asuitable grinding machine such as a jaw crusher or Brown mill. Thecoarse powder is then finely divided, for example, by a ball mill,vibration mill or jet mill using high-pressure nitrogen. Alternatively,fine division may be achieved by an atomizing method of ejecting a metalmelt through small nozzles under high-pressure gas as mist. The M¹ metalpowder has an average particle size equal to or less than 500 μm, morepreferably equal to or less than 300 μm, and even more preferably equalto or less than 100 μm. However, if the particle size is too small, thenthe influence of surface oxidation becomes noticeable, and handling isdangerous. Thus the lower limit of average particle size is preferablyequal to or more than 1 μm.

The other component of the powder mixture is an R² oxide which may beany of oxides of rare earth elements inclusive of Y and Sc, preferablyoxides containing Dy or Tb. The R² oxide powder has an average particlesize equal to or less than 100 μm, more preferably equal to or less than50 μm, and even more preferably equal to or less than 20 μm. The R²oxide is present in an amount of at least 10% by weight, preferably atleast 20% by weight, and more preferably at least 30% by weight of thepowder mixture. Less than 10% by weight of the R² oxide is too small forthe rare earth oxide to exert its mixing effect. The upper limit of theamount of the R² oxide is up to 99% by weight, especially up to 90% byweight.

After the powder mixture of the diffusion alloy powder or M¹ metalpowder and the R² oxide powder is disposed on the surface of the mothersintered magnet body, the mother sintered magnet body coated with thepowder mixture is heat treated in vacuum or in an atmosphere of an inertgas such as argon (Ar) or helium (He) at a temperature equal to or belowthe sintering temperature (designated Ts in ° C.) of the sintered magnetbody. This heat treatment is referred to as “diffusion treatment.” Thediffusion treatment causes the rare earth oxide in admixture with theintermetallic compound to be partially reduced, whereby elements R¹, R²,M¹, M² and T² in the powder mixture are diffused to grain boundaries inthe interior of the sintered magnet body and/or near grain boundarieswithin sintered magnet body primary phase grains in more amounts thanachievable in the prior art.

The powder mixture of the diffusion alloy powder or M¹ metal powder andthe R² oxide powder is disposed on the surface of the mother sinteredmagnet body, for example, by dispersing the powder mixture in water oran organic solvent to form a slurry, immersing the magnet body in theslurry, taking up the magnet body, and drying the magnet body by hot airdrying or in vacuum or in air. Spray coating is also possible. Theslurry may contain 1 to 90% by weight, and preferably 5 to 70% by weightof the powder mixture.

The conditions of diffusion treatment vary with the type and compositionof the powder mixture (including the type and composition of twocomponents) and are preferably selected such that elements R¹, R², M¹,M² and T² in the diffusion powder are enriched at grain boundaries inthe interior of the sintered magnet body and/or near grain boundarieswithin sintered magnet body primary phase grains. The temperature ofdiffusion treatment is equal to or below the sintering temperature(designated Ts in ° C.) of the sintered magnet body. If diffusiontreatment is effected above Ts, there arise problems that (1) thestructure of the sintered magnet body can be altered to degrade magneticproperties, and (2) the machined dimensions cannot be maintained due tothermal deformation. For this reason, the temperature of diffusiontreatment is equal to or below Ts° C. of the sintered magnet body, andpreferably equal to or below (Ts−10)° C. The lower limit of temperaturemay be selected as appropriate though the temperature is typically atleast 200° C., preferably at least 350° C., and more preferably at least600° C. The time of diffusion treatment is typically from 1 minute to 30hours. Within less than 1 minute, the diffusion treatment is notcomplete. If the treatment time exceeds 30 hours, the structure of thesintered magnet body can be altered, oxidation or evaporation ofcomponents inevitably occurs to degrade magnetic properties, or R¹, R²,M¹, M² and T² are not only enriched near grain boundaries in theinterior of the sintered body and/or grain boundaries within sinteredbody primary phase grains, but also diffused into the interior ofprimary phase grains. The preferred time of diffusion treatment is from1 minute to 10 hours, and more preferably from 10 minutes to 6 hours.

Through appropriate diffusion treatment, the constituent elements R¹ ,R², M¹, M² and T² in the powder mixture disposed on the surface of thesintered magnet body are diffused into the sintered magnet body whiletraveling mainly along grain boundaries in the sintered magnet bodystructure. This results in the structure in which R¹, R², M¹, M² and T²are enriched near grain boundaries in the interior of the sinteredmagnet body and/or grain boundaries within sintered magnet body primaryphase grains.

The permanent magnet thus obtained is improved in coercivity because thediffusion of R¹, R², M¹, M² and T² modifies the morphology near theprimary phase grain boundaries within the structure so as to suppress adecline of magnetocrystalline anisotropy at primary phase grainboundaries or to create a new phase at grain boundaries. Since theelements in the powder mixture have not diffused into the interior ofprimary phase grains, a decline of remanence is restrained. The magnetis a high performance permanent magnet.

After the diffusion treatment, the magnet may be further subjected toaging treatment at a temperature of 200 to 900° C. for augmenting thecoercivity enhancement.

EXAMPLE

Examples are given below for further illustrating the invention althoughthe invention is not limited thereto.

Example 1 and Comparative Examples 1 and 2

An alloy was prepared by weighing amounts of Nd, Co, Al and Fe metalshaving a purity of at least 99% by weight and ferroboron, high-frequencyheating in an argon atmosphere for melting, and casting the alloy melton a single roll of copper in an argon atmosphere, that is, stripcasting into a strip of alloy. The alloy consisted of 12.8 at % of Nd,1.0 at % of Co, 0.5 at % of Al, 6.0 at % of B, and the balance of Fe.This is designated alloy A. Alloy A was then subjected to hydrogendecrepitation by causing the alloy to absorb hydrogen, vacuum evacuatingand heating up to 500° C. for desorbing part of hydrogen. In this way,alloy A was pulverized into a coarse powder under 30 mesh.

Another alloy was prepared by weighing amounts of Nd, Dy, Fe, Co, Al andCu metals having a purity of at least 99% by weight and ferroboron,high-frequency heating in an argon atmosphere for melting, and castingthe alloy melt. The alloy consisted of 23 at % of Nd, 12 at % of Dy, 25at % of Fe, 6 at % of B, 0.5 at % of Al, 2 at % of Cu, and the balanceof Co. This is designated alloy B. Alloy B was ground on a Brown mill ina nitrogen atmosphere into a coarse powder under 30 mesh.

Next, 94 wt % of alloy A powder and 6 wt % of alloy B powder were mixedin a nitrogen-purged V-blender for 30 minutes. The powder mixture wasfinely pulverized on a jet mill using high-pressure nitrogen gas into afine powder having a mass median particle diameter of 4.1 μm. The finepowder was compacted in a nitrogen atmosphere under a pressure of about1 ton/cm² while being oriented in a magnetic field of 15 kOe. The greencompact was then placed in a sintering furnace where it was sintered inan argon atmosphere at 1,060° C. for 2 hours, obtaining a magnet blockof 10 mm×20 mm×15 mm (thick). Using a diamond grinding tool, the magnetblock was machined on all the surfaces into a shape having dimensions of4 mm×4 mm×2 mm (magnetic anisotropy direction). The machined magnet bodywas washed in sequence with alkaline solution, deionized water, acidsolution, and deionized water, and dried, obtaining a mother sinteredmagnet body which had the composition:Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0).

Tb and Al metals having a purity of at least 99% by weight were used andhigh-frequency melted in an argon atmosphere to form a diffusion alloyhaving the composition Tb₃₃Al₆₇ and composed mainly of an intermetalliccompound phase TbAl₂. The alloy was finely pulverized on a ball millusing an organic solvent into a fine powder having a mass medianparticle diameter of 8.6 μm. On electron probe microanalysis (EPMA), thealloy contained 94% by volume of the intermetallic compound phase TbAl₂.

The diffusion alloy Tb₃₃Al₆₇ powder was mixed with terbium oxide (Tb₄O₇)having an average particle size of 1 μm in a weight ratio of 1:1. Thepowder mixture was combined with deionized water in a weight fraction of50% to form a slurry, in which the mother sintered magnet body wasimmersed for 30 seconds under ultrasonic agitation. The magnet body waspulled up and immediately dried with hot air. The magnet body coveredwith the powder mixture was diffusion treated in an argon atmosphere at900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yieldinga magnet of Example 1.

Separately, the diffusion alloy Tb₃₃Al₆₇ powder having a mass medianparticle diameter of 8.6 μm alone was combined with deionized water in aweight fraction of 50% to form a slurry, in which the magnet body wasimmersed for 30 seconds under ultrasonic agitation. The magnet body waspulled up and immediately dried with hot air. The magnet body coveredwith the diffusion alloy powder was diffusion treated in an argonatmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, andquenched, yielding a magnet of Comparative Example 1. In the absence ofthe diffusion powder, only the mother sintered magnet body was similarlyheated treated in vacuum at 900° C. for 8 hours, yielding a magnet ofComparative Example 2.

Table 1 summarizes the composition of the mother sintered magnet body,diffusion rare earth alloy and diffusion rare earth oxide, and a mixingratio (by weight) of the diffusion powder in Example 1 and ComparativeExamples 1 and 2. Table 2 shows the temperature (° C.) and time (hr) ofdiffusion treatment and the magnetic properties of the magnets. It isseen that the magnet of Example 1 has a coercive force (Hcj) which isgreater by 90 kAm⁻¹ than that of Comparative Example 1 and a remanence(Br) which is higher by 8 mT than that of Comparative Example 1. Thecoercive force (Hcj) of the magnet of Example 1 is greater by 1,090kAm⁻¹ than that of Comparative Example 2 while a decline of remanence(Br) is only 5 mT.

TABLE 1 Diffusion powder mixture Mother sintered Rare earth Rare earthMixing ratio magnet body alloy oxide (by weight) Example 1Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) Tb₃₃Al₆₇ Tb₄O₇50:50 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) Tb₃₃Al₆₇ —Tb₃₃Al₆₇ alone Example 1 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) — — — Example 2

TABLE 2 Diffusion treatment Temperature Time Br Hcj (BH)_(max) (° C.)(hr) (T) (kAm⁻¹) (kJ/m³) Example 1 900 8 1.415 2,130 390 Comparative 9008 1.407 2,040 386 Example 1 Comparative 900 8 1.420 1,040 380 Example 2

Example 2 and Comparative Example 3

As in Example 1, a mother sintered magnet body having the composition:Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)al_(0.5)B_(6.0) was prepared.

Tb, Co, Fe and Al metals having a purity of at least 99% by weight wereused and high-frequency melted in an argon atmosphere to form adiffusion alloy having the composition Tb₃₅Fe₂₁Co₂₄Al₂₀. The alloy wasfinely pulverized on a ball mill using an organic solvent into a finepowder having a mass median particle diameter of 8.9 μm. On EPMAanalysis, the alloy contained intermetallic compound phases Tb(FeCoAl)₂,Tb₂(FeCoAl) and Tb₂(FeCoAl)₁₇, which summed to 87% by volume.

The diffusion alloy Tb₃₅Fe₂₁Co₂₄Al₂₀ powder was mixed with Tb₄O₇ havingan average particle size of 1 μm in a weight ratio of 1:1. The powdermixture was combined with deionized water in a weight fraction of 50% toform a slurry, in which the mother sintered magnet body was immersed for30 seconds under ultrasonic agitation. The magnet body was pulled up andimmediately dried with hot air. The magnet body covered with the powdermixture was diffusion treated in an argon atmosphere at 900° C. for 8hours, aged at 500° C. for 1 hour, and quenched, yielding a magnet ofExample 2.

In the absence of the diffusion powder, only the mother sintered magnetbody was similarly heat treated in vacuum at 900° C. for 8 hours,yielding a magnet of Comparative Example 3.

Table 3 summarizes the composition of the mother sintered magnet body,diffusion rare earth alloy and diffusion rare earth oxide, and a mixingratio (by weight) of the diffusion powder in Example 2 and ComparativeExample 3. Table 4 shows the temperature (° C.) and time (hr) ofdiffusion treatment and the magnetic properties of the magnets. It isseen that the coercive force (Hcj) of the magnet of Example 2 is greaterby 1,020 kAm⁻¹ than that of Comparative Example 3 while a decline ofremanence (Br) is only 4 mT.

TABLE 3 Diffusion powder mixture Mother sintered Rare earth Rare earthMixing ratio magnet body alloy oxide (by weight) Example 2Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0)Tb₃₅Fe₂₁Co₂₄Al₂₀ Tb₄O₇ 50:50 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) — — — Example 3

TABLE 4 Diffusion treatment Temperature Time Br Hcj (BH)_(max) (° C.)(hr) (T) (kAm⁻¹) (kJ/m³) Example 2 900 8 1.416 2,060 390 Comparative 9008 1.420 1,040 380 Example 3

Examples 3 to 55

As in Example 1, a series of mother sintered magnet bodies were coatedwith a different powder mixture of diffusion alloy and rare earth oxideand diffusion treated at a selected temperature for a selected time.Table 5 summarizes the composition of the mother sintered magnet body,diffusion rare earth alloy and rare earth oxide, and a mixing ratio (byweight) of the diffusion powder. Table 6 shows the temperature (° C.)and time (hr) of diffusion treatment and the magnetic properties of theresulting magnets. All the diffusion alloys contained at least 70% byvolume of intermetallic compounds.

TABLE 5 Diffusion powder mixture Mother sintered Rare earth Rare earthMixing ratio magnet body alloy oxide (by weight) Example 3Nd_(15.0)Fe_(bal)Co_(1.0)B_(5.4) Nd₃₅Fe₂₀Co₁₅Al₃₀ Tb₄O₇ 30:70 Example 4Nd_(15.0)Fe_(bal)Co_(1.0)B_(5.4) Nd₃₅Fe₂₅Co₂₀Si₂₀ Dy₂O₃ 60:40 Example 5Nd_(15.0)Fe_(bal)Co_(1.0)B_(5.4) Nd₃₃Fe₂₀Co₂₇Al₁₅Si₅ Nd₂O₃ 10:90 Example6 Nd_(11.0)Dy_(2.0)Tb_(2.0)Fe_(bal)Co_(1.0)B_(5.5) Nd₂₈Pr₅Al₆₇ Pr₂O₃90:10 Example 7 Nd_(16.5)Fe_(bal)Co_(1.5)B_(6.2) Y₂₁Mn₇₈Cr₁ Dy₂O₃ 50:50Example 8 Nd_(13.0)Pr_(2.5)Fe_(bal)Co_(2.8)B_(4.8) La₃₃Cu₆₀Co₄Ni₃ Tb₂O₃50:50 Example 9 Nd_(13.0)Pr_(2.5)Fe_(bal)Co_(2.8)B_(4.8) La₅₀Ni₄₉V₁ CeO₂70:30 Example 10 Nd_(13.0)Dy_(1.5)Fe_(bal)Co_(1.0)B_(5.9)La₃₃Cu_(66.5)Nb_(0.5) La₂O₃ 30:70 Example 11Nd_(16.5)Fe_(bal)Co_(3.0)B_(4.7) Ce₂₂Ni₁₄Co₅₈Zn₆ Tb₄O₇ 80:20 Example 12Nd_(16.5)Fe_(bal)Co_(3.0)B_(4.7) Ce₁₇Ni₈₃ CeO₂ 50:50 Example 13Nd_(17.3)Fe_(bal)Co_(3.5)B_(6.3) Ce₁₁Zn₈₉ Gd₂O₃ 50:50 Example 14Nd_(16.0)Dy_(1.5)Fe_(bal)Co_(4.5)B_(5.1) Pr₃₃Ge₆₇ Y₂O₃ 50:50 Example 15Nd_(12.2)Pr_(2.5)Fe_(bal)Co_(1.0)B_(5.3) Tb₃₃Al₆₀H₇ Dy₂O₃ 50:50 Example16 Nd_(14.5)Pr_(2.5)Fe_(bal)Co_(3.5)B_(5.6) Pr₃₃Al₆₆Zr₁ Tb₄O₇ 75:25Example 17 Nd_(13.0)Tb_(1.5)Fe_(bal)B_(5.5) Gd₃₂Mn₃₀Fe₃₁Nb₇ Dy₂O₃ 50:50Example 18 Nd_(12.0)Fe_(bal)Co_(1.0)B_(4.8) Gd₃₇Mn₄₀Co₂₀Mo₃ Tb₄O₇ 25:75Example 19 Nd_(13.0)Tb_(1.5)Fe_(bal)B_(5.5) Gd₂₁Mn₇₈Mo₁ Dy₂O₃ 40:60Example 20 Nd_(12.0)Fe_(bal)Co_(1.0)B_(4.8) Gd₃₃Mn₆₆Ta₁ Tb₄O₇ 50:50Example 21 Nd_(12.0)Pr_(2.7)Fe_(bal)Co_(2.5)B_(5.2) Tb₂₉Fe₄₅Ni₂₀Ag₆Yb₂O₃ 50:50 Example 22 Nd_(13.0)Pr_(2.0)Fe_(bal)Co_(2.5)B_(5.2) Tb₅₀Ag₅₀Tb₄O₇ 60:40 Example 23 Nd_(12.5)Dy_(3.0)Fe_(bal)Co_(0.7)B_(5.9) Tb₅₀In₅₀Dy₂O₃ 50:50 Example 24 Nd_(12.5)Pr_(2.5)Tb_(0.5)Fe_(bal)Co_(0.5)B_(5.0)Dy₃₁Ni₈Cu₅₅Sn₆ Tb₄O₇ 50:50 Example 25Nd_(10.0)Pr_(2.5)Dy_(2.5)Fe_(bal)Co_(0.6)B_(5.7) Dy₃₃Cu_(66.5)Hf_(0.5)Pr₂O₃ 50:50 Example 26 Nd_(13.0)Pr_(2.2)Fe_(bal)Co_(1.0)B_(5.3) Dy₃₃Fe₆₇Dy₂O₃ 50:50 Example 27 Nd_(12.8)Pr_(2.5)Tb_(0.2)Fe_(bal)Co_(1.0)B_(4.5)Er₃₃Mn₃₀Co₃₅Ta₂ Tb₄O₇ 50:50 Example 28Nd_(13.2)Pr_(2.5)Dy_(0.5)Fe_(bal)Co_(3.0)B_(6.3) Er₂₁Mn_(78.6)W_(0.4)Er₂O₃ 50:50 Example 29 Nd_(12.0)Tb_(3.5)Fe_(bal)Co_(3.5)B_(6.2)Yb₂₄Co₅Ni₆₉Bi₂ Tb₄O₇ 50:50 Example 30Nd_(13.0)Dy_(3.0)Fe_(bal)Co_(2.0)B_(4.8) Yb₅₀Cu₄₉Ti₁ Pr₂O₃ 50:50 Example31 Nd_(11.0)Tb_(3.5)Fe_(bal)Co_(3.5)B_(6.2) Yb₂₅Ni_(74.5)Sb_(0.5) Yb₂O₃50:50 Example 32 Nd_(15.5)Fe_(bal)Co_(1.0)B_(5.3) Nd₃₃Al₆₇ Tb₄O₇ 90:10Example 33 Nd_(15.1)Fe_(bal)Co_(1.0)B_(5.4) Nd₅₀Si₅₀ Dy₂O₃ 80:20 Example34 Nd_(14.8)Fe_(bal)Co_(1.0)B_(5.3) Nd₃₃Al₃₇Si₃₀ Dy₂O₃ 20:80 Example 35Nd_(11.8)Pr_(3.0)Fe_(bal)Co_(1.0)B_(5.3) Nd₃₄Al₆₁H₅ Tb₄O₇ 50:50 Example36 Nd_(12.3)Dy_(2.5)Fe_(bal)Co_(3.5)B_(5.4) Nd₂₇Pr₆Al₆₇ Tb₄O₇ 50:50Example 37 Nd_(15.1)Fe_(bal)Co_(1.0)B_(5.3) Dy₃₃Al₆₇ Dy₂O₃ 75:25 Example38 Nd_(13.6)Tb_(1.5)Fe_(bal)Co_(3.5)B_(5.2) Dy₃₃Ga₆₇ Tb₄O₇ 50:50 Example39 Nd_(15.1)Fe_(bal)Co_(1.0)B_(5.3) Tb₃₃Al₆₇ Dy₂O₃ 80:20 Example 40Nd_(13.5)Pr_(2.0)Dy_(2.0)Fe_(bal)Co_(2.5)B_(5.3) Tb₂₂Mn₇₈ Tb₄O₇ 50:50Example 41 Nd_(12.5)Pr_(2.5)Fe_(bal)Co_(1.0)B_(5.3) Tb₃₃Co₆₇ Dy₂O₃ 50:50Example 42 Nd_(19.0)Fe_(bal)Co_(3.0)B_(5.4) Y₁₀Co₁₅Zn₇₅ Y₂O₃ 70:30Example 43 Nd_(18.0)Fe_(bal)Co_(2.5)B_(6.6) Y₆₈Fe₂In₃₀ Tb₄O₇ 50:50Example 44 Nd_(18.0)Fe_(bal)Co_(3.0)B_(5.4) Y₁₁Zn₈₉ Dy₂O₃ 80:20 Example45 Nd_(13.5)Pr_(1.5)Dy_(0.8)Fe_(bal)Co_(2.5)B_(4.5) La₃₂Co₄Cu₆₄ Tb₄O₇50:50 Example 46 Nd_(13.5)Pr_(1.5)Dy_(0.8)Fe_(bal)Co_(2.5)B_(4.5)La₃₃Cu₆₇ Pr₂O₃ 50:50 Example 47 Nd_(20.0)Fe_(bal)Co_(5.5)B_(4.1)Ce₂₆Pb₇₄ Tb₄O₇ 40:60 Example 48 Nd_(15.2)Fe_(bal)Co_(1.0)B_(5.3)Ce₅₆Sn₄₄ CeO₂ 50:50 Example 49Nd_(15.5)Dy_(2.5)Tb_(0.5)Fe_(bal)Co_(2.6)B_(4.4) Pr₃₃Fe₃C₆₄ Dy₂O₃ 50:50Example 50 Nd_(12.5)Dy_(2.0)Tb_(0.5)Fe_(bal)Co_(3.8)B_(6.2) Pr₅₀P₅₀Nd₂O₃ 50:50 Example 51 Nd_(12.7)Pr_(2.5)Dy_(0.6)Fe_(bal)Co_(1.4)B_(5.6)Gd₅₂Ni₄₈ Tb₄O₇ 70:30 Example 52Nd_(13.1)Pr_(1.5)Tb_(0.5)Fe_(bal)Co_(2.8)B_(6.3) Gd₃₇Ga₆₃ Dy₂O₃ 60:40Example 53 Nd_(15.3)Dy_(0.6)Fe_(bal)Co_(1.0)B_(4.9) Er₃₂Mn₆₇Ta₁ Nd₂O₃50:50 Example 54 Nd_(14.5)Pr_(1.0)Dy_(0.5)Fe_(bal)Co_(2.8)B_(4.6)Yb₆₈Pb₃₂ Tb₄O₇ 50:50 Example 55Nd_(12.0)Pr_(1.5)Dy_(0.5)Fe_(bal)Co_(4.2)B_(5.3) Yb₆₉Sn₂₉Bi₂ Yb₂O₃ 80:20

TABLE 6 Diffusion treatment Temperature Time Br Hcj (BH)max (° C.) (hror min) (T) (kAm⁻1) (kJ/m³) Example 3 780 8 h 1.404 2,032 385 Example 4880 8 h 1.419 1,992 390 Example 5 820 6 h 1.416 2,036 389 Example 6 7505 h 1.411 1,987 388 Example 7 930 10 h 1.343 1,008 343 Example 8 780 5 h1.367 1,225 354 Example 9 890 7 h 1.388 1,219 363 Example 10 820 8 h1.432 1,052 396 Example 11 450 12 h 1.348 920 349 Example 12 840 6 h1.353 940 343 Example 13 400 5 h 1.327 1,052 340 Example 14 830 5 h1.328 1,890 341 Example 15 820 8 h 1.412 2,130 385 Example 16 850 8 h1.371 2,048 363 Example 17 960 10 h 1.410 1,785 376 Example 18 940 6 h1.454 1,620 398 Example 19 920 5 h 1.411 1,615 381 Example 20 860 5 h1.452 1,748 396 Example 21 920 10 h 1.414 1,672 379 Example 22 920 6 h1.412 1,910 384 Example 23 940 12 h 1.405 1,955 381 Example 24 870 12 h1.404 1,930 382 Example 25 860 10 h 1.409 1,870 383 Example 26 850 8 h1.408 2,060 382 Example 27 1,020 8 h 1.376 1,610 362 Example 28 980 12 h1.368 1,521 363 Example 29 320 15 min 1.397 1,580 370 Example 30 380 25min 1.351 1,430 354 Example 31 410 40 min 1.430 1,243 390 Example 32 7908 h 1.404 2,070 382 Example 33 820 10 h 1.421 2,034 388 Example 34 910 5h 1.416 2,095 386 Example 35 760 8 h 1.417 2,100 386 Example 36 770 8 h1.421 2,120 387 Example 37 830 8 h 1.410 2,130 384 Example 38 760 3 h1.414 2,140 386 Example 39 880 8 h 1.416 2,170 389 Example 40 660 20 h1.353 1,860 354 Example 41 860 8 h 1.414 2,110 386 Example 42 450 12 h1.317 1,290 326 Example 43 1,030 2 h 1.286 1,346 309 Example 44 450 8 h1.332 1,211 334 Example 45 660 14 h 1.350 1,407 347 Example 46 620 12 h1.347 1,314 344 Example 47 520 10 h 1.203 1,305 276 Example 48 460 14 h1.361 1,120 350 Example 49 860 30 h 1.278 1,258 312 Example 50 360 40min 1.412 1,185 368 Example 51 960 2 h 1.390 1,545 366 Example 52 850 30min 1.415 1,410 382 Example 53 700 10 h 1.373 1,099 355 Example 54 75012 h 1.351 1,460 346 Example 55 420 10 h 1.448 1,020 396

Example 56 and Comparative Example 4

An alloy was prepared by weighing amounts of Nd, Co, Al and Fe metalshaving a purity of at least 99% by weight and ferroboron, high-frequencyheating in an argon atmosphere for melting, and casting the alloy melton a single roll of copper in an argon atmosphere, that is, stripcasting into a strip of alloy. The alloy consisted of 12.8 at % of Nd,1.0 at % of Co, 0.5 at % of Al, 6.0 at % of B, and the balance of Fe.This is designated alloy A. Alloy A was then subjected to hydrogendecrepitation by causing the alloy to absorb hydrogen, vacuum evacuatingand heating up to 500° C. for desorbing part of hydrogen. In this way,alloy A was pulverized into a coarse powder under 30 mesh.

Another alloy was prepared by weighing amounts of Nd, Dy, Fe, Co, Al andCu metals having a purity of at least 99% by weight and ferroboron,high-frequency heating in an argon atmosphere for melting, and castingthe alloy melt. The alloy consisted of 23 at % of Nd, 12 at % of Dy, 25at % of Fe, 6 at % of B, 0.5 at % of Al, 2 at % of Cu, and the balanceof Co. This is designated alloy B. Alloy B was ground on a Brown mill ina nitrogen atmosphere into a coarse powder under 30 mesh.

Next, 94 wt % of alloy A powder and 6 wt % of alloy B powder were mixedin a nitrogen-purged V-blender for 30 minutes. The powder mixture wasfinely pulverized on a jet mill using high-pressure nitrogen gas into afine powder having a mass median particle diameter of 4 μm. The finepowder was compacted in a nitrogen atmosphere under a pressure of about1 ton/cm² while being oriented in a magnetic field of 15 kOe. The greencompact was then placed in a sintering furnace where it was sintered inan argon atmosphere at 1,060° C. for 2 hours, obtaining a magnet blockof 10 mm×20 mm×15 mm (thick). Using a diamond grinding tool, the magnetblock was machined on all the surfaces into a shape having dimensions of4 mm×4 mm×2 mm (magnetic anisotropy direction). The machined magnet bodywas washed in sequence with alkaline solution, deionized water, acidsolution, and deionized water, and dried, obtaining a mother sinteredmagnet body which had the composition:Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0).

Al and Co metals having a purity of at least 99% by weight were used andhigh-frequency melted in an argon atmosphere to form a diffusion alloyhaving the composition Al₅₀Co₅₀ and composed mainly of an intermetalliccompound phase AlCo. The alloy was finely pulverized on a ball millusing an organic solvent into a fine powder having a mass medianparticle diameter of 8.9 μm. On EPMA analysis, the alloy contained 94%by volume of the intermetallic compound phase AlCo.

The diffusion alloy Al₅₀Co₅₀ powder was mixed with terbium oxide (Tb₄O₇)having an average particle size of 1 μm in a weight ratio of 1:1. Thepowder mixture was combined with deionized water in a weight fraction of50% to form a slurry, in which the mother sintered magnet body wasimmersed for 30 seconds under ultrasonic agitation. The magnet body waspulled up and immediately dried with hot air. The magnet body coveredwith the powder mixture was diffusion treated in an argon atmosphere at900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yieldinga magnet of Example 56.

Separately, terbium oxide having an average particle size of 1 μm alonewas combined with deionized water in a weight fraction of 50% to form aslurry, in which the magnet body was immersed for 30 seconds underultrasonic agitation. The magnet body was pulled up and immediatelydried with hot air. The coated magnet body was diffusion treated in anargon atmosphere at 900° C. for 8 hours, aged at 500° C. for 1 hour, andquenched, yielding a magnet of Comparative Example 4.

Table 7 summarizes the composition of the mother sintered magnet body,diffusion alloy and diffusion rare earth oxide, and a mixing ratio (byweight) of the diffusion powder mixture in Example 56 and ComparativeExample 4. Table 8 shows the temperature (° C.) and time (hr) ofdiffusion treatment and the magnetic properties of the magnets. It isseen that the coercive force (Hcj) of the magnet of Example 56 isgreater by 90 kAm⁻¹ than that of Comparative Example 4 while a declineof remanence (Br) is only 3 mT. The coercive force (Hcj) of the magnetof Example 56 is greater by 1,040 kAm⁻¹ than that of previousComparative Example 2 while a decline of remanence (Br) is only 4 mT.

TABLE 7 Diffusion powder mixture Mother sintered Diffusion Rare earthMixing ratio magnet body alloy oxide (by weight) Example 56Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) Al₅₀Co₅₀ Tb₄O₇50:50 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) — Tb₄O₇ Tb₄O₇alone Example 4 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) — — — Example 2

TABLE 8 Diffusion treatment Temperature Time Br Hcj (BH)_(max) (° C.)(hr) (T) (kAm⁻¹) (kJ/m³) Example 56 900 8 1.416 2,080 390 Comparative900 8 1.419 1,990 393 Example 4 Comparative 900 8 1.420 1,040 380Example 2

Example 57 and Comparative Example 5

As in Example 56, a mother sintered magnet body having the composition:Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) was prepared.

Ni and Al metals having a purity of at least 99% by weight were used andhigh-frequency melted in an argon atmosphere to form a diffusion alloyhaving the composition Ni₂₅Al₇₅ and composed mainly of an intermetalliccompound phase NiAl₃. The alloy was finely pulverized on a ball millusing an organic solvent into a fine powder having a mass medianparticle diameter of 9.3 μm. On EPMA analysis, the alloy contained 94%by volume of the intermetallic compound phase NiAl₃.

The diffusion alloy Ni₂₅Al₇₅ powder was mixed with terbium oxide (Tb₄O₇)having an average particle size of 1 μm in a weight ratio of 1:1. Thepowder mixture was combined with deionized water in a weight fraction of50% to form a slurry, in which the mother sintered magnet body wasimmersed for 30 seconds under ultrasonic agitation. The magnet body waspulled up and immediately dried with hot air. The magnet body coveredwith the powder mixture was diffusion treated in an argon atmosphere at900° C. for 8 hours, aged at 500° C. for 1 hour, and quenched, yieldinga magnet of Example 57. In the absence of the diffusion powder mixture,the sintered magnet body alone was heat treated in vacuum at 900° C. for8 hours, yielding a magnet of Comparative Example 5.

Table 9 summarizes the composition of the mother sintered magnet body,diffusion alloy and diffusion rare earth oxide, and a mixing ratio (byweight) of the diffusion powder mixture in Example 57 and ComparativeExample 5. Table 10 shows the temperature (° C.) and time (hr) ofdiffusion treatment and the magnetic properties of the magnets. It isseen that the coercive force (Hcj) of the magnet of Example 57 isgreater by 1,010 kAm⁻¹ than that of Comparative Example 5 while adecline of remanence (Br) is only 4 mT.

TABLE 9 Diffusion powder mixture Mother sintered Diffusion Rare earthMixing ratio magnet body alloy oxide (by weight) Example 57Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) Ni₂₅Al₇₅ Tb₄O₇50:50 ComparativeNd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.4)Cu_(0.1)Al_(0.5)B_(6.0) — — — Example 5

TABLE 10 Diffusion treatment Temperature Time Br Hcj (BH)_(max) (° C.)(hr) (T) (kAm⁻¹) (kJ/m³) Example 57 900 8 1.416 2,050 390 Comparative900 8 1.420 1,040 380 Example 5

Examples 58 to 96

As in Example 56, a series of mother sintered magnet bodies were coatedwith a different powder mixture of diffusion alloy (or metal) and rareearth oxide and diffusion treated at a selected temperature for aselected time. Table 11 summarizes the composition of the mothersintered magnet body, diffusion alloy and rare earth oxide, and a mixingratio (by weight) of the diffusion powder mixture. Table 12 shows thetemperature (° C.) and time (hr) of diffusion treatment and the magneticproperties of the resulting magnets. All the diffusion alloys containedat least 70% by volume of intermetallic compounds.

TABLE 11 Diffusion powder mixture Mother sintered Diffusion Rare earthMixing ratio magnet body alloy or metal oxide (by weight) Example 58Nd_(15.0)Fe_(bal)Co_(1.0)B_(5.4) Mn₂₇Al₇₃ Tb₄O₇ 30:70 Example 59Nd_(12.0)Pr_(3.0)Fe_(bal)Co_(3.0)B_(5.2) Ni₂₅Al₇₅ Dy₂O₃ 90:10 Example 60Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.0)B_(6.0) Al Tb₄O₇ 50:50 Example 61Nd_(14.3)Dy_(1.2)Fe_(bal)Co_(2.0)B_(5.3) Cr_(12.5)Al_(87.5) Nd₂O₃ 20:80Example 62 Nd_(13.8)Tb_(0.7)Fe_(bal)Co_(1.0)B_(5.5) Co₃₃Si₆₇ Pr₂O₃ 70:30Example 63 Nd_(15.8)Fe_(bal)Co_(1.5)B_(5.3) Mn₂₅Al₂₅Cu₅₀ Tb₄O₇ 50:50Example 64 Nd_(14.4)Dy_(0.8)Tb_(0.3)Fe_(bal)Co_(1.0)B_(5.4) Fe₅₀Si₅₀CeO₂ 60:40 Example 65 Nd_(18.2)Fe_(bal)Co_(4.0)B_(5.3)Fe_(49.9)C_(0.1)Si₅₀ La₂O₃ 30:70 Example 66Nd_(13.3)Dy_(0.5)Fe_(bal)Co_(2.0)B_(6.0) Si Tb₄O₇ 50:50 Example 67Nd_(17.6)Fe_(bal)Co_(3.5)B_(4.2) Cr_(12.5)Al_(87.5) Tb₄O₇ 50:50 Example68 Nd_(15.6)Fe_(bal)Co_(1.0)B_(6.8) Mn₆₇P₃₃ Dy₂O₃ 50:50 Example 69Nd_(12.0)Fe_(bal)Co_(2.0)B_(6.0) Ti₅₀Cu₅₀ Gd₂O₃ 50:50 Example 70Nd_(12.9)Dy_(1.0)Fe_(bal)Co_(2.0)B_(6.0) Cu Dy₂O₃ 50:50 Example 71Nd_(15.2)Fe_(bal)Co_(1.0)B_(5.5) V₇₅Sn₂₅ Tb₄O₇ 75:25 Example 72Nd_(14.3)Fe_(bal)B_(6.1) Cr₆₇Ta₃₃ Dy₂O₃ 50:50 Example 73Nd_(14.8)Fe_(bal)Co_(3.0)B_(5.4) Cu₇₅Sn₂₅ Y₂O₃ 50:50 Example 74Pr_(15.0)Fe_(bal)Co_(6.5)B_(5.3) Cu₇₀Zn₅Sn₂₅ Er₂O₃ 60:40 Example 75Nd_(13.8)Dy_(0.8)Fe_(bal)Co_(2.0)B_(6.2) Zn Dy₂O₃ 50:50 Example 76Nd_(15.8)Pr_(1.5)Fe_(bal)Co_(2.5)B_(5.2) Ga₄₀Zr₆₀ Tb₄O₇ 60:40 Example 77Nd_(13.5)Dy_(1.0)Fe_(bal)Co_(2.0)B_(6.0) Ga Tb₄O₇ 50:50 Example 78Nd_(15.2)Fe_(bal)Co_(3.0)B_(5.3) Cr₇₅Ge₂₅ Yb₂O₃ 50:50 Example 79Nd_(14.0)Dy_(0.8)Fe_(bal)Co_(3.0)B_(6.0) Ge Dy₂O₃ 50:50 Example 80Nd_(14.6)Pr_(2.0)Dy_(0.8)Fe_(bal)Co_(2.0)B_(5.3) Nb₃₃Si₆₇ Dy₂O₃ 50:50Example 81 Pr_(13.7)Dy_(1.0)Fe_(bal)Co_(1.0)B_(5.4) Al₇₃Mo₂₇ Pr₂O₃ 40:60Example 82 Nd_(15.0)Fe_(bal)Co_(1.0)B_(6.4) Ti₅₀Ag₅₀ Nd₂O₃ 60:40 Example83 Nd_(13.8)Dy_(1.0)Fe_(bal)Co_(1.0)B_(5.8) Ag Tb₄O₇ 50:50 Example 84Nd_(14.3)Fe_(bal)Co_(1.0)B_(5.3) In₂₅Mn₇₅ Tb₄O₇ 50:50 Example 85Nd_(13.9)Fe_(bal)B_(5.6) Hf₃₃Cr₆₇ Dy₂O₃ 70:30 Example 86Nd_(15.2)Fe_(bal)Co_(1.0)B_(5.6) Cr₂₅Fe₅₅W₂₀ Tb₄O₇ 50:50 Example 87Nd_(15.1)Yb_(0.2)Fe_(bal)Co_(1.0)B_(4.8) Ni₅₀Sb₅₀ Er₂O₃ 50:50 Example 88Nd_(15.7)Fe_(bal)Co_(5.0)B_(6.9) Ti₈₀Pb₂₀ Tb₄O₇ 60:40 Example 89Nd_(14.6)Fe_(bal)Co_(1.0)B_(5.3) Mn₂₅Co₅₀Sn₂₅ La₂O₃ 70:30 Example 90Nd_(14.9)Fe_(bal)Co_(0.7)B_(5.3) Co₆₀Sn₄₀ Tb₄O₇ 50:50 Example 91Nd_(14.6)Fe_(bal)Co_(1.5)B_(5.5) V₇₅Sn₂₅ Er₂O₃ 30:70 Example 92Nd_(12.8)Pr_(2.0)Fe_(bal)Co_(3.0)B_(5.6) Sn Tb₄O₇ 50:50 Example 93Nd_(14.2)Fe_(bal)Co_(0.5)B_(5.6) Cr₂₁Fe₆₂Mo₁₇ Tb₄O₇ 50:50 Example 94Nd_(15.0)Dy_(0.6)Fe_(bal)Co_(0.1)B_(4.1) Bi₄₀Zr₆₀ Dy₂O₃ 40:60 Example 95Nd_(15.2)Fe_(bal)Co_(3.5)B_(6.4) Ni₅₀Bi₅₀ Yb₂O₃ 50:50 Example 96Nd_(12.0)Pr_(3.0)Fe_(bal)Co_(2.0)B_(6.1) Bi Dy₂O₃ 50:50

TABLE 12 Diffusion treatment Temperature Time Br Hcj (BH)_(max) (° C.)(hr or min) (T) (kAm⁻¹) (kJ/m³) Example 58 790 3 h 1.413 2,087 387Example 59 810 3 h 30 min 1.407 2,187 384 Example 60 850 8 h 1.414 1,980388 Example 61 760 1 h 1.380 1,928 368 Example 62 820 2 h 30 min 1.4232,042 394 Example 63 770 5 h 1.394 2,223 373 Example 64 820 4 h 1.4021,861 383 Example 65 940 12 h 1.298 1,904 328 Example 66 870 8 h 1.4151,930 389 Example 67 1,060 28 h 1.284 1,713 319 Example 68 380 15 min1.358 1,512 353 Example 69 680 8 h 1.476 1,498 409 Example 70 820 8 h1.417 1,820 390 Example 71 940 5 h 1.414 1,816 387 Example 72 1,020 10 h1.426 1,896 393 Example 73 650 8 h 1.420 1,641 387 Example 74 600 10 h1.406 1,689 379 Example 75 760 8 h 1.403 1,760 379 Example 76 840 5 h1.355 1,940 351 Example 77 870 8 h 1.415 1,950 389 Example 78 850 7 h1.420 1,816 390 Example 79 880 8 h 1.411 1,890 387 Example 80 1,000 10 h1.358 1,896 355 Example 81 770 1 h 1.417 2,085 386 Example 82 760 4 h1.404 1,530 380 Example 83 920 8 h 1.413 1,910 386 Example 84 630 13 h1.446 1,780 401 Example 85 960 7 h 1.433 1,620 394 Example 86 920 15 h1.413 1,940 385 Example 87 750 6 h 1.381 1,537 363 Example 88 920 5 h1.369 1,338 355 Example 89 640 6 h 1.424 1,418 391 Example 90 880 40 min1.414 2,040 383 Example 91 1,020 10 h 1.420 1,450 387 Example 92 730 5 h1.408 1,820 383 Example 93 880 15 h 1.454 1,800 406 Example 94 510 20 h1.346 1,430 343 Example 95 360 5 min 1.392 1,211 362 Example 96 420 15min 1.382 1,510 358

Japanese Patent Application Nos. 2011-102787 and 2011-102789 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

What is claimed is:
 1. A method for preparing a rare earth permanentmagnet, comprising the steps of: disposing a powder mixture on a surfaceof a sintered magnet body having the composition R_(a)T¹ _(b)M_(c)B_(d)wherein R is at least one element selected from rare earth elementsinclusive of Y and Sc, T¹ is one or both of Fe and Co, M is at least oneelement selected from the group consisting of Al, Si, C, P, Ti, V, Cr,Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, andBi, B is boron, “a,” “b,” “c” and “d” indicative of atomic percent arein the range: 12≤a≤20, 0≤c≤10, 4.0≤d≤7.0, the balance of b, anda+b+c+d=100, the powder mixture comprising an M¹ powder wherein M¹ is atleast one element selected from the group consisting of Al, Si, C, P,Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb,Hf, Ta, W, Pb, and Bi, having an average particle size of up to 500 μm,and at least 10% by weight of an R² oxide wherein R² is at least oneelement selected from rare earth elements inclusive of Y and Sc, havingan average particle size of up to 100 μm, and heat treating the sinteredmagnet body having the powder mixture disposed on its surface at atemperature lower than or equal to the sintering temperature of thesintered magnet body in vacuum or in an inert gas, for causing theelements R² and M¹ in the powder mixture to diffuse to grain boundariesin the interior of the sintered magnet body and/or near grain boundarieswithin the sintered magnet body primary phase grains.
 2. The method ofclaim 1 wherein the heat treating step includes heat treatment at atemperature from 200° C. to (Ts−10)° C. for 1 minute to 30 hours whereinTs represents the sintering temperature of the sintered magnet body. 3.The method of claim 1 wherein the disposing step includes dispersing thepowder mixture in an organic solvent or water, immersing the sinteredmagnet body in the resulting slurry, taking up the sintered magnet body,and drying for thereby covering the surface of the sintered magnet bodywith the powder mixture.
 4. The method of claim 1 wherein the sinteredmagnet body has a shape including a minimum portion with a dimensionequal to or less than 20 mm.
 5. A rare earth permanent magnet, which isprepared by disposing a powder mixture on a surface of a sintered magnetbody having the composition R_(a)T¹ _(b)M_(c)B_(d) wherein R is at leastone element selected from rare earth elements inclusive of Y and Sc, T¹is one or both of Fe and Co, M is at least one element selected from thegroup consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr,Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, B is boron, “a,” “b,” “c”and “d” indicative of atomic percent are in the range: 12≤a≤20, 0≤c≤10,4.0≤d≤7.0, the balance of b, and a+b+c+d=100, the powder mixturecomprising an M¹ powder wherein M¹ is at least one element selected fromthe group consisting of Al, Si, C, P, Ti, V, Cr, Mn, Ni, Fe, Co, Cu, Zn,Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi, having anaverage particle size of up to 500 μm, and at least 10% by weight of anR² oxide wherein R² is at least one element selected from rare earthelements inclusive of Y and Sc, having an average particle size of up to100 μm, and heat treating the sintered magnet body having the powdermixture disposed on its surface at a temperature lower than or equal tothe sintering temperature of the sintered magnet body in vacuum or in aninert gas, wherein the elements R² and M¹ in the powder mixture arediffused to grain boundaries in the interior of the sintered magnet bodyand/or near grain boundaries within the sintered magnet body primaryphase grains so that the coercive force of the rare earth permanentmagnet is increased over the original sintered magnet body.